The present invention relates to a switched-mode power supply having a transformer, a switching transistor connected in series with a primary winding of the transformer, a control circuit and a secondary-side regulating stage. Switched-mode power supplies of this type are used in appliances for consumer electronics, for example.
To be able to turn appliances having a switched-mode power supply on and off using a remote control, it is necessary to keep them in a constant standby mode in order to turn them on. However, this means that the switched-mode power supply is constantly consuming power. To keep the power consumption of the switched-mode power supply as low as possible in standby operation, switched-mode power supplies with a burst mode have been developed, for example, or a separate power supply unit is used just for standby operation. EP-A 0 803 966 discloses a power supply unit, for example, in which a relatively large switched-mode power supply is used just for normal operation and a smaller switched-mode power supply is used for standby operation. In this arrangement, the two switched-mode power supplies are coupled to one another such that the larger switched-mode power supply is regulated by means of the small switched-mode power supply during normal operation, and the small switched-mode power supply oscillates using a dedicated oscillator in standby operation.
The invention is based on the object of specifying a switched-mode power supply of the type mentioned in the introduction which has a very low power consumption.
The switched-mode power supply of the invention has a transformer, a switching transistor connected in series with a primary winding of the transformer, a primary-side control circuit and a secondary-side regulating stage. In this arrangement, the control circuit is used to drive the switching transistor. The secondary-side regulating circuit is used to drive a coupling element, which is used to transmit a regulating signal from the secondary to the primary. A first switch is situated between the control input of the switching transistor and a primary-side operating voltage, and a second switch is situated between the regulating stage and a secondary-side operating voltage, with the two operating voltages being able to be disconnected by means of the two switches using a single control signal. In this case, the isolating element transmits both the regulating information for the primary-side control circuit and the turn-off signal for the first switch.
In this case, the control signal for turning off the two switches is applied to a control input of the second switch, and, when this switch is off, the secondary-side regulating stage is disconnected at the same time and the first switch is turned off via the coupling element. The coupling element is preferably an optocoupler driven on the secondary side by a transistor stage of the secondary-side regulating stage, at which both the switched-mode power supply""s output voltage which is to be regulated and, via the second switch, the control signal information are present.
If the second switch is turned off by the control signal, the regulating stage and the optocoupler become completely currentless and consume no further power. As a result of this, the optocoupler is off on the primary side, which means that the first switch is also turned off, so that both the switching transistor and the control circuit become non-live. In this situation, the switched-mode power supply is completely currentless except for the starting circuit, so that the power consumption is below 0.2 W in this state.